Add code formatting to the pipeline.

[tomasssalles]

... with black, probably.

[tomasssalles]

• Run with auto-fix when pushing to master.
• Run before unit tests?
• Config in pyproject.toml
• Badge with (not quite but almost)

.. image:: https://img.shields.io/badge/code%20style-black-000000.svg
    :target: https://github.com/psf/black

• Verbose
• Multiple target-versions target-version = ['py36', 'py37', 'py38', 'py39']
• Print version with black --version
• Line-length default

[tomasssalles]

Issues to be solved:

• Tests might be running without the reformatting commit because the checkout action uses some hash defined when the workflow starts (probably).
• Combined coverage cannot be pushed again because it is missing the reformatting commit.
• Simply doing a git pull might not work... In the case of a pull request, there seems to be a temporary commit used only for the workflow run which merges the feature branch into main (makes sense). Not even sure what branch we're in, if any.
• Take a look at the docs workflow to see some options for the checkout action (though it might not help).

To do in code:

• Find in tests all the places where a datetime was stretched over several lines by black (look for uses of 'timezone' I think). See if storing the timezone in a variable is enough for black to put everything back into one line again.

[tomasssalles]

A nice solution would be to run black in a separate workflow. But can we ensure the right workflow order?